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PROGRAM

Date:

3 December 2015

Time:

15:00 hrs

Location:

Gorterzaal (Oort 173), Niels Bohrweg 2, Leiden

ABSTRACT: In theory, Cooper-pairs can exist with finite momentum Q and thereby generate a spatially modulating Cooper-pair density with wavelength 2π/Q. Such a ‘pair density wave’ (PDW) state has never been observed directly in any superconductor. However, the recent discovery of a charge density wave state in cuprate superconductors has motivated several contemporary microscopic theories in which the cuprate pseudogap phase contains a PDW state. To study this issue we developed a nanometer resolution, millikelvin operating temperature, scanned Josephson (Cooper-pair) tunneling microscope (SJTM). We image Cooper-pair tunneling from a d-wave superconducting STM tip at millikelvin temperatures to the Cooper-pair condensate of underdoped Bi2Sr2CaCu2O8. We demonstrate its condensate visualization capabilities directly using the Cooper-pair density variations surrounding Zn impurity atoms and those caused by the well known crustal supermodulation in Bi2Sr2CaCu2O8. We then image the Cooper-pair condensate with nanometer resolution throughout large fields of view for Bi2Sr2CaCu2O8 samples within the pseudogap regime. Unprocessed, these images show clear Cooper-pair density modulations oriented along the Cu-O bond directions. By using Fourier analysis we detect the direct signature of a Cooper-pair density modulation at wavevectors QP≈(0.25,0)2π/a0;(0,0.25)2π/a0; the amplitude of these modulations is ~ 3% of the homogeneous amplitude and their form factor exhibits primarily s/s’-symmetry. This precise PDW phenomenology is predicted for cuprates based on general order-parameter symmetry principles, and by several contemporary microscopic theories for the cuprate pseudogap phase. This technique for imaging a Cooper-pair condensate also opens the prospect of condensate visualizations in other cuprates, and in pnictides and unconventional superconductors.

NB: if you want to meet with Mohammad Hamidian before his talk, contact his host, Milan Allan (allan@physics.leidenuniv.nl).